Slony-I is a "master to multiple slaves" replication
system for PostgreSQL
supporting cascading (e.g. - a node can feed
another node which feeds another node...) and failover.

The big picture for the development of Slony-I is
that it is a master-slave replication system that includes
all features and capabilities needed to replicate large
databases to a reasonably limited number of slave
systems.

Slony-I is a system designed for use at data centers and
backup sites, where the normal mode of operation is that all
nodes are available.

A fairly extensive "admin guide" comprising material in
the Git tree may be found here. There
is also a local copy.

As observed up front, the streaming replication built into
PostgreSQL, which has progressed since its beginnings in version 8.1,
can satisfy some users' replication requirements. For those with
requirements that are not compatible with its strictures, replication
systems like Slony-I will continue to be useful.

PostgreSQL 9.4 introduced Logical Replication, which uses
logical decoding of WAL data as an alternative to trigger-based data
capture. In the long run, this strategy seems likely to be more
performant than Slony, and to eventually obsolesce Slony for many use
cases not involving upgrades between PostgreSQL versions.
However, Logical Replication/Logical Decoding will only support
recent versions of PostgreSQL (and inter-PostgreSQL-version support
may be aspirational moreso than actual), so if you have a database
running on 9.3 or earlier, there is a likelihood that
short-outage-upgrades will require something like Slony that runs
against a diverse set of versions of PostgreSQL.